1. Field of the Invention
The present invention relates to coded orthogonal frequency division multiplexing (COFDM), and more particularly to interleaving for wireless local area networks (WLANs) using COFDM.
2. Description of Related Art
Coded orthogonal frequency division multiplexing (COFDM) is a type of radio modulation that splits a single high-speed data stream into many much slower data streams. Each is transmitted over its own frequency-division sub-carrier and then reassembled with the others at the receiver to reconstruct the original data streams. The carriers are evenly spaced with one another, and are said to be orthogonal. Such orthogonality of the carriers greatly simplifies modulation and demodulation of so many carriers, and packs all of them together without an increase in bandwidth.
In COFDM systems, the performance and data reliability can be improved in the face of noise and other interference by spreading the data bits. In a transmitter, after encoding and prior to modulation, the coded data is “interleaved” to avoid the carrying of successive or adjacent coded bits in adjacent subcarriers. Swapping of the order of significant bits is also required so that adjacent coded bits are alternately assigned to less and more significant bits of the constellation during modulation. Thus, both interleaving of coded data and bit order mapping of the interleaved data are required.
The reverse de-interleaving operation that occurs in the receiver after demodulation also includes a bit order mapping.
A typical interleaver implementation uses a memory as a buffer. However, a design that uses a memory buffer may get clogged down with the data rates. As an example, the data rate in an IEEE 802.11a compliant system in a sixty-four-symbol quadrature amplitude modulation 64QAM) configuration can reach 72-MHz, requiring fast memory devices. Thus it is desired to reduce the data rate requirements of buffers used in a buffered interleaver.
Furthermore, it is desired to combine the interleaving or de-interleaving operation with the required bit order mapping, e.g., bit reversal needed for the modulation or demodulation process.
For more information on the IEEE 802.11 and IEEE 802.11a standards, see: ANSI/IEEE Std 802.11, 1999 Edition (ISO/IEC 8802-11:1999) Local and metropolitan area networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and IEEE Std 802.11a-1999 [ISO/IEC 8802-11:1999/Amd 1:2000(E)] (Supplement to IEEE Std 802.11, 1999 Edition) Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band. The standards are available on the Internet at several locations, including from the IEEE (www.IEEE.org) and in particular at http://grouper.ieee.org/groups/802/11/index.html.